It may be effective to coat the surface of a base material powder with a carbon film having a uniform thickness of about several nanometers (nm) for performing modification of the base material powder, and such a base material powder is used, for example, in an intermediate production process or as an intermediate raw material for the production of MgB2 superconductor, a positive electrode material for a lithium ion battery, a photocatalyst or the like. In this case, various substances have been known as the sources of carbon that coats the base material powder; however, upon the addition of an aromatic hydrocarbon, the aromatic hydrocarbon is decomposed at the time of heat treatment, and hydrogen is generated. There is a possibility that this hydrogen may cause failure in the applications of final manufactured products of the base material powder. Furthermore, a method of coating the surface of base material powder particles with carbon by a gas phase method has also been suggested; however, since the control of the carbon coating layer is difficult, and since a gas phase method is used, there is a problem that production of base material powders in large quantities is difficult (highly expensive).
Next, in regard to MgB2 superconductor, which is one of the applications of the base material powder having a carbon nanocoating layer, MgB2 superconductor has a critical temperature Tc that is higher compared to practical superconducting materials, and also has advantages such as follows for practical use.
i) On the occasion of passing a large superconducting current from one crystal grain to a neighboring crystal grain, it is considered unnecessary to align the directions of crystal grains (orientation) such as in the case of high temperature oxide superconductors;
ii) MgB2 superconductor is abundant in resources, and the raw materials are relatively inexpensive;
iii) MgB2 superconductor is mechanically tough; and
iv) MgB2 superconductor is lightweight.
Therefore, MgB2 superconductor is considered to be promising as a practical useful material, and research and development thereof is currently underway.
On the other hand, MgB2 superconductor has a problem that the upper critical magnetic field HC2 is low. In this regard, it has been reported that when some of B sites are substituted by carbon (C), the HC2 is increased to a large extent. The most common method for substituting B sites with C is a method of adding SiC powder to a mixed raw material powder of Mg and B, and heat-treating the mixture. Furthermore, a method of adding an aromatic hydrocarbon to raw material powders of Mg and B is also effective, and enhancement of the Jc characteristics in a high magnetic field are achieved by substituting some B sites in MgB2 crystals with C through the addition of an aromatic hydrocarbon (see Patent Literatures 1 to 3).
However, in the addition of an aromatic hydrocarbon, the aromatic hydrocarbon is decomposed at the time of heat treatment, and hydrogen is generated. There is a possibility that this hydrogen generation may cause defects during the production of a long superconducting wire. On the other hand, a method of coating the surface of B powder particles with C by a gas phase method has also been reported. That is, if methane gas is introduced when B nanopowder is produced by a rf plasma method using BCl3 as a raw material, carbon-coated B nanopowder is obtained. However, this method has a problem that Cl remains as an impurity, control of the carbon coating layer is difficult, and production of B powder in large quantities is difficult (very expensive) because the method is a gas phase method.
Furthermore, since lithium ion secondary batteries have high voltages and high energy densities, lithium ion secondary batteries are widely used for portable electronic equipment such as mobile phones and laptop computers, and power supplies to be mounted in vehicles. Regarding the positive electrode of a lithium ion secondary battery, various substances such as lithium cobaltate and lithium manganate are available; however, among these, LiFePO4 is a material to which attention is paid as a positive electrode material for large-sized batteries for the following reasons.
(i) The substance does not include a rare metal;
(ii) the substance is harmless and is highly safe; and
(iii) the substance has satisfactory cycle characteristics.
However, electrical conductivity is lower by about a number of the order of 1,000 to a number of the order of 100,000 compared to other positive electrode materials, and in order to increase the electrical conductivity, LiFePO4 nanopowder particles are used, and the nanopowder particles have a carbon nanocoating in which acetylene black or the like is used, on the surface.
Therefore, one of the most important technologies for the commercialization of LiFePO4 is the deposition of a conductive agent such as a carbon nanocoating on the surface of LiFePO4 particles (see, for example, Non Patent Literature 4). Examples of the method of producing a carbon coating on LiFePO4 include a method of adding a polyvinyl chloride powder in a solid phase method, and a method of using methanol (see, for example, Patent Literatures 4 and 5, and Non Patent Literature 7). However, these prior art technologies need to use a solvent or to use a kiln having a rotating function, and there is a problem that all of these technologies are not simple in terms of process, and cannot be said to be operable at low cost.
On the other hand, it may be effective to cover the surface of an electrode active material with a carbon film having a uniform thickness of about several nanometers (nm), for performing modification of the base material powder that constitutes an electrode active material, and for the production of a positive electrode material for a lithium ion battery or the like, the base material powder is used in an intermediate production process or as an intermediate raw material. In this case, various substances have been known as the sources of carbon that coats the base material powder; however, upon the addition of an aromatic hydrocarbon, the aromatic hydrocarbon is decomposed at the time of heat treatment, and hydrogen is generated. There is a possibility that this hydrogen may cause failure in the applications of final manufactured products of the base material powder. Furthermore, a method of coating the surface of base material powder particles with carbon by a gas phase method has also been suggested; however, since the control of the carbon coating layer is difficult, and since a gas phase method is used, there is a problem that production of base material powders in large quantities is difficult (highly expensive).